US West Texas Intermediate crude futures fell 41 cents, or 0.6 percent, to $73.38 a barrel by 00:53 GMT.
The contract was not traded on Friday, because US markets were closed for the Christmas holiday.
Brent crude, the global benchmark, rose 40 cents, or 0.5%, to $76.54 a barrel, after settling at 0.92% on Friday.
Both contracts jumped by between three and four percent last week, after early data indicated that the mutated Omicron of the Corona virus, may cause a milder level of disease.
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